This invention relates to power amplifiers, and more particularly relates to predistortion circuits and methods for compensating for nonlinearities within the amplification process.
Radio frequency (RF) power amplifiers are widely used to transmit signals in communications systems. Typically a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of amplitude, phase and/or frequency, causing the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes the carrier itself is not sent since it is not essential to the communication of the information.
A signal which varies in amplitude will suffer distortion during amplification if the amplifier does not exhibit a linear amplitude characteristic. Perfect linearity over a wide range of amplitude is difficult to realize in practice. The signal will also suffer distortion if the phase shift introduced by the amplifier (1) varies with the signal""s amplitude, or (2) is not linear over the range of frequencies present in the signal. The distortion introduced typically includes intermodulation of the components of the input signal. In addition to appearing within the bandwidth of the signal, such distortion products typically extend outside the bandwidth originally occupied by the signal, potentially causing interference in adjacent channels. Although filtering can be used to remove the unwanted out of band distortion, filtering is not always practical, especially if the amplifier is required to operate on several different frequencies.
A typical amplifier becomes significantly nonlinear at a small fraction of its maximum output capacity. In order to maintain linearity, the amplifier is therefore operated at an input and output amplitude which is low enough that the signals to be amplified are in a part of the transfer characteristic which is substantially linear. In this mode of operation, known as xe2x80x9cbacked offxe2x80x9d, the amplifier has a low supplied power to transmitted power conversion efficiency. For example, a xe2x80x9cClass Axe2x80x9d amplifier operating in this mode may have an efficiency of only 1%. In addition to wasting power, amplifiers operated in a backed off mode tend to be large and expensive.
One method for compensating for an amplifier""s nonlinearities is known as predistortion. With traditional predistortion, an inverse model of the amplifier""s nonlinear transfer characteristic is formed and is then applied to the low level signal at the input of the amplifier. The input signal is thus predistorted in a manner that is equal to and opposite from the distortion introduced during amplification, so that the amplified signal appears undistorted. To account for variations in the amplifier""s transfer characteristic, the inverse model is updated based on a real-time observation of the amplifier""s input and output signals.
One problem with existing predistortion methods is that they are generally based on the assumption, known as the memoryless AM-AM and AM-PM assumption, that (a) the nonlinear response of the amplifier is independent of the instantaneous frequency of the stimulating waveform, and (b) the nonlinear response of the amplifier is independent of previous amplifier input stimulus. Unfortunately, (a) and (b) generally do not hold true for wideband applications. As a result, existing predistortion techniques do not produce satisfactory results within wideband systems.
Another problem with existing predistortion techniques is that they fail to accurately take into account memory effects (effects of past stimulus) within the AM-AM and AM-PM distortion characteristic. Such memory effects are often caused by fluctuations in amplifier transistor die temperatures which occur as the result of variations in the amplitude of the signal being amplified. Failure to accurately predict and account for such memory effects can produce poor results.
The present invention addresses the above and other problems with existing predistortion schemes.
The present invention provides a wideband predistortion system and associated methods for compensating for non-linear characteristics of a power amplifier, including the amplifier""s frequency and time dependent AM-AM and AM-PM distortion characteristics. The system preferably comprises a data structure in which each element stores a set of compensation parameters (preferably including FIR filter coefficients) for predistorting the wideband input signal. The parameter sets are preferably indexed within the data structure according to multiple signal characteristics, such as instantaneous amplitude and integrated signal envelope, each of which corresponds to a respective dimension of the data structure.
To predistort the input transmission signal, an addressing circuit digitally generates a set of data structure indices by measuring the input transmission signal characteristics by which the data structure is indexed. In one embodiment, a data structure index is also generated from the output of a transistor die temperature sensor. On each sample instant, the indexed set of compensation parameters is loaded into a compensation circuit that predistorts the input transmission signal. The compensation circuit, which may be implemented in application-specific circuitry, preferably includes a finite impulse response (FIR) filter, and may also include an IQ modulator correction circuit.
The sets of compensation parameters are generated and written to the data structure by an adaptive processing component, which may be implemented using a programmed microprocessor or digital signal processor. The adaptive processing component generates the compensation parameter sets during regular amplifier operation by performing a non-real-time analysis of captured amplifier input and output signals. The adaptive processing component also preferably implements a state machine for controlling the overall operation of the amplifier system.
The adaptive processing component also implements a system identification process for measuring the characteristics of the power amplifier and generating initial sets of compensation parameters. As part of this process, stimulation signals are applied to the amplifier to measure various characteristics of the amplifier, including amplitude-dependent and frequency-dependent characteristics. The measured characteristics are used to generate a non-linear model of the amplifier. An input signal is then applied to both the amplifier and its model while monitoring a difference between the respective outputs, and the parameters of the model are adaptively adjusted until an error floor is reached. The level of complexity of the model is then increased, and the adaptive process repeated, until a desired level of model accuracy is reached. The model is then used to generate initial sets of compensation parametersxe2x80x94preferably using a direct inversion and/or adaptive process.
In one specific embodiment of, and application for, the invention, the predistortion architecture is used to compensate for nonlinearities in each amplification chain of an antenna array system. A compensation circuit of the type described above is provided along each amplification chain. However, rather than providing separate adaptive processing components for each amplification chain, a single adaptive processing component is used on a time-shared basis to generate and update the compensation parameters for all of the amplification chains.
In another specific embodiment of, and application for, the invention, the amplification chain includes a power splitter that feeds multiple nonlinear amplifiers. The nonlinear amplifiers are individually controlled (e.g., turned ON and OFF) to conserve power, such as during low traffic conditions. The amplification chain thus has multiple operating points, each of which corresponds to a particular combination of amplifier states. In this embodiment, the data structure is expanded, such as by adding an additional dimension, to store sets of compensation parameters for each operating point of the amplification chain.
Additional inventive features are set forth below.